Microwaves are not ovens
I watched a show on the food network today, the topic of which was kitchen gadgets. One of these gadgets was a device by Whirlpool that is supposed to combine the speed and convenience of a microwave with the baking/browning power of the conventional oven. The result: the MicrOven.
A blend between microwave and oven, this word is particularly interesting because the original name (and still the best?) for the cooking devices that use microwave radiation is microwave oven. The coining of that term clearly did nothing to change the overall semantic coverage of the word oven — it still refers to the conventional kind, and it can’t be used on its own to refer to a microwave. I might hesitate if asked whether a “microwave oven” was a type of oven. Another example would (for some people) be short pants (aka shorts), which may or may not be a subtype of “pants”. [Note that I'm pretty sure that simple microwave came to be used to refer to the devices rather soon after they were commonly called microvawe ovens. At least this is what the OED leads one to believe.]
Contrast this with toaster oven. Now, the basic way that regular ovens and toaster ovens work is often the same (though I’ve never seen a gas-powered toaster oven). If someone asked if I had an oven, but all I had was a toaster oven, then I might say, “well, no, but I have a toaster oven — good enough?” But if all I had was a microwave oven, it would probably seem preposterous to say, “well, no, but I have a microwave — that’s just as good, right?” At the very least, there would be more situations where it would be preposterous, because the overlap in function between conventional ovens and microwaves is much smaller than with toaster ovens.
I think the more common situation for N-N compounds is that what the compound denotes is a subtype of the right-hand noun (ski lifts, correspondence courses, video games, conflict diamonds). Closer to the area of recent technology, consider mobile/cell phones, which are considered types of phones, and which can be called such. Or think of the (electronic) card key, which has managed to hold on to the term key since it has the same basic function as normal keys, even though it works in a rather different way. (On the other hand, they are more often called key cards, perhaps partly because they are often seen along with other “cards,” like ID cards. But this usage has not seemed to change the meaning of plain “card” - if asked to empty my wallet of cards, I might think, “but I don’t have any cards in my wallet.”)
Returning to talk of function, a microwave oven has, at least in my life, a rather different function from other sorts of devices that you stick things into in order to heat them up. In fact, I don’t think there are many dishes that I could prepare equally well in either device. Yes, I could make microwave brownies and melt butter in a conventional oven: but why would I? [okay, my microwave is currently broken, so if I didn't have any pots...] I would think that most people think somewhat similarly. And for those people for whom shorts are not a type of “pants,” perhaps the same could be said. And if I thought like mulling it over more and thinking of more compounds like this, maybe I could see if such is the case for a majority of the microwave oven-type compounds.
[Looking at my shelf of syntax and semantics books, I just thought of some nasty pseudo-examples: trace theory (see Skeptical Linguistics Essays) and, to give equal time to multiple camps, cognitive linguistics (see...uh...some people, I'm sure). Oh, and Chinese grammar (okay, that was out of line)]